SPRG Seminars
February 7, 2012:
"The Statistics of Solar Microflares, the Coronal Heating Problem, and FOXSI"
Steven Christe, Space Science Laboratory, University of California, Berkeley
The coronal heating problem has been an outstanding issue in solar physics since Edlen and Grotrian identified Fe IX and Ca XIV lines in the solar spectrum which implied a coronal temperature of 1 MK. Since the photosphere is at temperature of 6000 K, this raises the interesting puzzle of how the coronal temperature can be maintained, the so-called coronal heating problem. Since Parker (1988) suggested that continuous flaring at small scales, so-called nanoflares, may be the solution there has been much interest in studying the smallest possible energy releases on the Sun to evaluate their contribution to coronal heating. Large flares are well observed and are known to not produce enough energy even when extrapolated to small sizes beyond observation limits. There must therefore be a break up in the frequency distribution of flares for nanoflares to be a viable mechanism to heat the corona. This raises the question of whether nanoflares or small flares are simply scaled down versions of large flares or something different altogether.
RHESSI is the most sensitive solar hard x-ray observatory currently available and has been observing the Sun for 10 years. It has observed tens of thousands of flares and therefore can provide important information about small flares especially when combined with other observations such as in the EUV or radio. The future for investigating weak solar x-ray brightenings is through the combination of grazing incidence optics combined with position sensitive solid state detectors which can offer 100 times more sensitivity than is currently available. The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space program which will fly soon and will make the most sensitive x-rays observations of the quiet Sun to measure the nonthermal energy content of nanoflares.